In this paper, the tool geometry of the plane strain punch stretching (hereafter PSS) test was optimized in order to assure the reliability of the test. Considering several factors for optimization of the tool geometry, a computer-simulation technique using the three-dimensional finite element method (FEM) was used. Three design variables - the punch length, punch crown and punch corner radius - are chosen to be optimized ac­cording to the Taguchi`s experimental technique with the L9 orthogonal array. The optimum condition to en­sure the plane strain mode over the overall area of the specimen was clarified. The simulation results have been confirmed by experiment.